1. Field of the Invention
The present invention relates to transporting substrates and, more particularly, to moving substrates into and out of areas with limited individual holding areas.
2. Prior Art
Substrate processing apparatus, commonly referred to as cluster tools, are known that include a module for supplying substrates from the exterior into a main processing section or chamber from which they are transferred into substrate processing modules communicating with the main section. The main chamber may be maintained in a vacuum and has a substrate transport for moving substrates among the processing modules, which transport may be of the type of transfer device described in PCT patent publication No. WO 94/23911. The processing modules may be of various forms familiar to the art. The substrate supply module is connected to a front end of the main section and typically has a frame, a substrate transport, and means for holding two substrate cassettes. The front end of the main section has two load locks that function as compartments for transporting the substrates between the vacuum chamber and the supply module 16, i.e., between a vacuum environment and an atmospheric pressure environment. An external or atmospheric robot transports the substrates from the cassettes to the load locks, and an internal or vacuum chamber robot transports the substrates from the load locks to the processing modules. When the substrate processing is finished, the vacuum chamber robot transports the substrates from the processing modules back to the load locks and the atmospheric robot transports the substrates from the load locks back to the cassettes. Typically, the load locks are indexing load locks which have numerous substrate support shelves and an elevator mechanism to move the shelves up and down. The shelves in the load locks may be as many as 30, depending upon the corresponding number of substrates held in a single cassette. The external robot loads a full cassette of substrates into each load lock. The internal robot loads and unloads the substrates between the load locks and the processing modules and then returns the processed substrates back to their cassettes. In the prior art apparatus, a computer controller is programmed to move the vacuum robots such that a substrate moved from a first location, such as a shelf in one of the the load locks, will be returned to the same location after being transported to a second location. Recently, substrate processing apparatus are being manufactured to process newer larger substrates, such as 300 mm diameter semiconductor wafers or flat panel display substrates which could be as large as a square meter. Indexing load locks for such large substrates can hold a large quantity of substrates and have the advantage of providing very good substrate throughput. Large size substrates must also be relatively slowly exposed to environmental change in the load locks in order to prevent undesired effects such as vapor condensation on the substrates. Indexing load locks are again of advantage as they can effectively compensate or the longer load lock environment change time to retain good substrate throughput. However, indexing load locks are very expensive, and thus a problem is presented regarding how to maintain good substrate throughput, but nonetheless reduce the costs associated with large substrate indexing load locks.
3. Object
It is therefore an object of the present invention to provide a substrate processing apparatus which can achieve a comparable substrate throughput to that of an apparatus with large substrate indexing load locks while being of considerably lower cost.